1. Field of the Invention
The present invention relates to a method for automatically controlling output power, and more particularly, to a method for automatically controlling output power of a pick-up head of an optical drive with an APC loop.
2. Description of the Prior Art
In recent years, along with the increasing operating capability of the computer system combined with the development of Internet technology, users have widely made use of the computer system as the multi-media audiovisual medium and made use of the computer as a bridge for connecting with a network to access all kinds of information. Due to the increasing need of the data storage quantity, various tools and apparatuses for storing data immediately become popular. Since the optical disk has the advantages of compactness, large storage capacity, and inexpensiveness, related products became very attractive. Recently, the functions of the optical drive (such as a CD-RW drive) have increased, and the reading quality and access speed of the optical drive have been improved continuously. Moreover, in addition to the original specification of CD, the new specification of DVD appears with much larger capacity and the same physical volume with CD. Nowadays, the optical drive has become the standard equipments of the computer system.
The CD-RW drive access data according to the optical principles, therefore the reading and writing operations depend on a pick-up head, which is usually a laser head. During the reading process, the CD-RW drive will set the output power of the output laser of the pick-up head to a predetermined value to set the wavelength of the output laser to a constant value so that the wavelength of reflected light is equal to a value of a optical signal that a sensor of the CD-RW drive can detect. The optical disk stores the data by the way of pressing or recording some concaves, convexes, or special membranes with various optical characteristics on the surface of the optical disk so that the optical sensor can distinguish a plurality of different wavelengths of reflected light to store the data with the digital form. During the writing process, a CD-RW drive also will set the output power of the output laser of the pick-up head to a predetermined value to set the wavelength of the output laser to a constant value so that the pick-up head can identify the parameters of the membranes on the surface of the optical disk and control the laser to output a plurality of wavelengths continuously according to the digital data to be written onto the optical disk. Therefore, the digital data can be pressed and recorded onto the optical disk.
Please refer to FIG. 1. During the reading process and writing process, in order to make the CD-RW drive maintain the output power of the laser pick-up head at a predetermined value without fluctuating with the changes of the environment such as the temperature, the prior art usually makes use of an APC loop 10 in a CD-RW drive as shown in FIG. 1 to form a feedback closed loop with a pick-up head 20 for stabilizing the output power. The APC loop 10 comprises a drive circuit 18, a comparator circuit 14, a sensor 12, and a signal source 16. The drive circuit 18 is electrically connected to the pick-up head 20 for driving the pick-up head 20. The comparator circuit 14 comprises a first input port, a second input port, and an output port. The comparator circuit 14 compares two signals respectively transmitted from the first input port and the second input port to generate a corresponding comparative signal y. The output port is electrically connected to the drive circuit 18 for outputting the comparative signal y to the drive circuit 18. The sensor 12 is used to detect the output power of the pick-up head 20 to generate a corresponding detecting signal ε and to input the detecting signal ε to the first inputs port of the comparator circuit 14. The signal source 16 is used to provide a signal γ to the second input port of the comparator circuit 14, and the signal γ represents the expected value of the output power of the pick-up head 20 of the CD-RW drive. The sensor 12 creates the signal E which then feedbacks to the comparator circuit 14, and the APC loop 10 makes use of the comparator circuit 14 to compare the feed-backed signal E with the signal γ representing the expected value of output power to generate the comparative signal y for controlling the output power of the pick-up head 20. Making use of the feedback control mechanism and designing the APC loop 10 with proper parameters can maintain the output power of the pick-up head 20 at an anticipant value. The user can insert various signal amplification circuits or power amplification circuits among the components of the APC loop 10 (such as inserting amplification circuits between the output port of the comparator circuit 14 and the drive circuit 18) according to practical needs. Moreover, the comparator circuit 14 can be achieved with various circuit configurations, and generally the comparator circuit 14 comprises an operational amplifier 22, a capacitor 24, and two resistors 26 and 28 that are connected as shown in FIG. 1. The signal source 16 usually is a digital signal-processing (DSP) chip for generating a digital signal that is transformed through a D/A converter.
However, the APC loop 10 has a very serious drawback when the output power of the pick-up head 20 is to be changed in the CD-RW drive. That is, the APC loop 10 needs to take a period of time to reach steady state. Please refer to FIG. 2. FIG. 2 is a schematic diagram showing how the signal γ, the comparative signal y, the detecting signal ε (as shown in FIG. 1), and the voltage drop Vc of the capacitor 24 vary with the time dimension t. Please notice that regarding the parameters of the components of the APC loop 10, when the signal γ is set as γ1 and the APC loop 10 reaches the steady state, the comparative signal y can be set as γ1, the detecting signal ε can be set as γ1, and the voltage drop Vc is (γ1−y1). When the signal γ is γ2 and the APC loop 10 reaches the steady state, the comparative signal y is y2, the detecting signal ε is γ2, and the voltage drop Vc becomes (γ2−y2). When that CD-RW drive wants to raise the output power of the pick-up head 20 from a lower value to a higher value, the signal y will be switched from γ1 to γ2 at time t1. At this time, the voltages of all nodes in the APC loop 10 will be shifted from original steady-state values to new steady-state values. However, due to the effect of capacitance in the APC loop 10 (such as the capacitor 24 that provides most of the effect of capacitance in the APC loop 10 as shown in FIG. 1), the new steady state will be reached after the effective capacitor is charged. As shown in FIG. 2, the voltage drop Vc is (γ1−y1) at time t1, and then at time t2 the voltage drop Vc enters a steady-state value (γ2−y2) after charging process. Similarly, the comparative signal y is y1+(γ2−γ1) at time t1, and at time t2 the comparative signal y reaches a steady-state value y2 after charging process. The detecting signal ε is γ2 at time t1, and then enters a steady-state value γ2 at time t2. When that CD-RW drive wants to adjust the output power of the pick-up head 20 from a higher value to a lower value, the signal γ will be shifted from γ2 to γ1 at time t3. At this time, the voltages of all nodes in the APC loop 10 willbe shifted from original steady-state values to new steady-state values. However, due to the effect of capacitance in the APC loop 10, the new steady state will be reached after the effective capacitor is discharged. As shown in FIG. 2, the voltage drop Vc is (γ2−y2) at time t3, and then enters a steady-state value (γ1−y1) at time t4 after discharging process. Similarly, the comparative signal y is y2−(γ2−γ1) at time t3, and then enter a steady-state value y1 at time t4 after discharging process. The detecting signal ε is γ1 at time t3, and then reaches a steady-state value γ1 at time t4 after discharging process.
The above-mentioned effect of capacitance in the APC loop resulting from the charging/discharging process toward the effective capacitor will cause a period of time of unsteady state, and the unsteady state will do harm to the operations of the CD-RW drive. During the writing process, when the reading speed of a buffer is higher than the writing speed, the CD-RW drive must stop recording until the register enters the idle status. Because the long period of time of unsteady state leads to the destabilization of the output power, bug data are easily generated in the connecting point. During the reading process, the long period of time of unsteady state easily leads to the servo failure. For example, the tracking servo or the focusing servo may be out of control during the reading process.